1. Field of the Invention
This invention relates in general to magnetic recording, and in particular to improvements therein.
2. Description Relative of the Prior Art
In the magnetic recording of information signals on a magnetic medium such as magnetic tape, a magnetic record core having a high reluctance gap therein is (usually) employed. Magnetic flux, corresponding to a signal to be recorded, is generated (by suitable means) in the record core and, as the flux bridges the record core gap, it extends outwardly from the gap. By appropriately placing the record core in contact with the medium, the fringe field flux extending from the gap licks the magnetic medium and, remanently, records signal information in the medium. To play back the signal information recorded in the medium, a gapped magnetic playback core is (usually) employed. The playback core (which in some instances may be the record core used for playback purposes) gathers, at its gap, the signal flux which was recorded in the medium and (assuming there is relative motion between the playback core and medium while such flux is being gathered) produces an electric signal across a coil inductively linked to the playback core. In connection with the matter of signal playback, it is well known to make the playback gap about half as long as the record gap in order to play back short wavelength recorded signals, "wavelength", in this regard, referring to the distance, along the recording on the recording medium, between successive similarly magnetized portions of the medium. That is, the length of the playback gap defines the shortest signal wavelength which can be recovered during playback of recorded signals.
It is believed to be appropriate at this point of address the matter of gap length definition: the term "gap length" as used throughout this specification, whether in connection with record or playback cores, should not be taken to mean the physical distance between the magnetic core poles at opposite ends of a gap; and which distance is sometimes referred to as the "physical gap length", or "optical gap length", or "mechanical gap length". Rather, "gap length" as used throughout this specification, unless otherwise indicated, is taken to mean "magnetic gap length", which is the effective gap length. The magnetic gap length of a magnetic head may be determined, for example, by (1) recording, with a reference magnetic head of an appropriate type, a set of test signals of various wavelengths on a magnetic medium, and (2) determining the wavelength at which a head under test experiences a null corresponding to a ratio of gap length to wavelength which equals one--as discussed by Athey, Magnetic Tape Recording, NASA Publication No. SP-5038, page 66--such determined wavelength equalling the magnetic gap length of the head under test. In support of the proposition that there is a distinction between the magnetic and physical gap lengths, Athey further states (same page):
". . . the effective length of the gap, judged from the position of the response nulls, is longer than the physical gap."
In a test as described above, it will be appreciated that a record head, the magnetic gap length of which is to be determined, is employed in a playback mode to play back the indicated test signals.
Just as it is well known to employ a playback gap length which is optimally short, it has also been known in the art to employ, where practical, a record gap length which is relatively long: the theory is that the trailing edge of a gap with respect to a relatively moving recording medium is the primary head part associated with the recording process; such being the case, it has been reasoned that by making the record gap length relatively large, record flux will significantly extend outwardly from the record gap, and thereby cause the magnetic medium to record, efficiently, throughout its depth, albeit in a region downstream of the record gap. Admittedly the length of the record gap affects the dimensions of the recording zone within the magnetic medium: the shorter the record gap length, the smaller the recording zone for the same recording current. This is an important consideration, since it correlates during playback with the matter of resolving short wavelength signals. Certainly, in the case where a narrow band of relatively short wavelength signals is to be recorded, and played back, the record gap can be relatively short. Where, however, long wavelength recording is involved, as in the case of a broadband recorder, a recording impasse results. This point was addressed by C. D. Mee in his definitive text, The Physics of Magnetic Recording, 1964, John Wiley and Sons, Inc., New York, page 245, where he states:
". . . the recording zones for a narrow gap recording head are shown . . . it can be seen that the narrowest recording zone 1) is achieved, but when sufficient field is applied to magnetize the tape fully, the recording zone is just as large as that from the wide gap head. Hence no advantage is to be gained by use of extremely narrow gaps for wideband recording." PA1 "The operating gap in the reproduce head, however, is usually considerably smaller than that in the record head . . ."
Similarly, Athey, in his above-referred to work (page 23) states:
Others in the field have also made this point, e.g., Gordon White, Video Recording Record and Replay Systems, Crane, Russak and Company, Inc., 1972, and Charles E. Lowman, Magnetic Recording, McGraw-Hill Book Company, 1972.
Implicit in the above reference to wideband recording is a recognition that recorders, such a quadruplex and helical scan video recorders, can employ the same relatively narrow gap head(s), for record and playback purposes, because they are relatively short wavelength/narrow band recorders. Indeed, as will appear in the representative table below, such recorders are all adapted to record less than a single octave:
______________________________________ Minimum Required Recorder Wavelength Magnetic Gap Length ______________________________________ Sony Betamax 36.mu."-50.mu." 36.mu." U-Matic 47.mu."-68.mu." 47.mu." EIAJ-1 60.mu."-87.mu." 60.mu." Quadruplex 100.mu."-167.mu." 100.mu." .mu." = microinch ______________________________________
It will be appreciated that some prior art video recorders, e.g. the Betamax recorder, record, by means of the same head, a narrow band luminance signal and a separate narrow band chroma signal such that there is more than one octave between the extreme ends of the chroma and luminance bandwiths. This is not "wideband recording" as contemplated by the invention, and is in actuality the recording of a pair of discrete narrow band signals each of which is no more than about an octave in bandwith. Evidence that such recordings are discrete may be appreciated from the fact that each such recording requires its own separate equalization. For wideband recording, as, for example, in an instrumentation recorder, or in the direct recording of audio and video information at relatively slow writing speeds, the usual practice is to employ a wide gap head for recording, and a narrow gap head for playback. Typical instrumentation recorders, according to the above reference by Lowman, page 35, for example, employ record and reproduce gaps as follows:
______________________________________ Response Record Gap Reproduce Gap ______________________________________ A. 100 kHz @ 60 ips 500.mu." 250.mu." B. 600 kHz @ 120 ips 500.mu." 80.mu." C. 2 mHz @ 120 ips 250.mu." 25.mu." ______________________________________
And, if the shortest recoverable wavelength is calculated for the above A, B and C responses, it will be appreciated that the reproduce gaps which are stated are, in fact, the physical gap lengths, the upper requisite limits to the sizes of magnetic gap lengths being, respectively, 600.mu.", 200.mu.", and 60.mu.".